<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .container {
      display: flex;
    }
    .left {
      width: 200px;
      height: 200px;
      background-color: aqua;
    }

    .right {
      width: 200px;
      height: 200px;
      background-color: aquamarine;
    }

    .mid {
      background-color: cadetblue;
      /* height: 200px; */
      flex: 1;
      /* margin-left: 200px;
      margin-right: 200px; */
    }
  </style>
</head>

<body>
  <div class="container">
    <div class="left"></div>
    <div class="mid"></div>
    <div class="right"></div>
  </div>
</body>

</html>